The present invention relates to a backlight device, a liquid crystal display apparatus, and a light deflecting sheet suitably applicable to a light source of the liquid crystal display apparatus and a room lighting system, for example.
Recently, displays highly reduced in thickness, such as an LCD (liquid crystal display) and a PDP (plasma display panel), have been proposed for a display of a television receiver instead of a CRT (cathode ray tube), and have been put into practical use. In particular, the liquid crystal display including a liquid crystal display panel has become widespread following the driving with low electrical power consumption and reduction in price of a large-sized liquid crystal display panel, and has been pushed ahead with research and development.
In such a liquid crystal display, a backlight system is the mainstreamer in that color images are displayed by two-dimensionally illuminating a transmission liquid crystal display panel having a color filter from the rear.
As the light source of the backlight system, a cold cathode fluorescent lamp (CCFL) radiating white light by using a fluorescent lamp and a light emitting diode (LED) are regarded as promising (see Patent Document 1, for example).
In particular, by the development of a blue-color LED, LEDs respectively irradiating red-color light, green-color light, and blue-color light, which are three primary colors, appear all together, so that white light with high chromatic purity can be obtained by mixing the red-color light, the green-color light, and the blue-color light emitted from these LEDs. Thus, using these LEDs as the light source of the backlight device increases the chromatic purity of light has passed the liquid crystal display panel, thereby widely spreading a color reproduction range in comparison with the CCFL. Furthermore, using a light emitting diode (LED) chip with high output power largely improves the brightness of the backlight device.
When using LEDs of the three primary colors (the red-color light, the green-color light, and the blue-color light) in the light source of such a backlight device, if the LEDs are arranged directly below a light emitting plane, i.e., they are a directly under type of diode, brightness non-uniformity and color non-uniformity may be liable to generate because the LED is a point light source.
Means for achieving this problem may include increasing the distance between the LED and a diffusion plate; however, the thickness of the backlight device becomes very large in comparison with that including the CCFL. In the liquid crystal display, the further reduction in weight and thickness is essential so that the backlight device also requires further reducing its thickness.
Then, a method for reducing the thickness is proposed in that by a light guide plate, light from the light source arranged on one side of the light guide plate is multipath-reflected to form a surface light source.
In a side-light type structure of the backlight device including the light guide plate, in which a number of light sources are arranged on one side of the light guide plate for increasing the brightness, a structure is proposed in that a concave structure having a triangular cross-section is formed on the bottom surface of the light guide plate opposite to the light outgoing part (see Patent Document 2, for example).
In order to uniformize the in-plane distribution of the brightness, various technical contrivances are proposed in shapes and arrangements of the diffusing plate and a prism sheet, for example. The prism sheet generally used widely at present may include BEF™ from Sumitomo 3M Limited and DIAART™ from MITSUBISHI RAYON Co., LTD. The former prism sheet, as shown in the perspective view of FIG. 47 as an example, is a light deflecting sheet 150 having a triangular-pole concavo-convex shaped plane continuously formed on one surface, and it has a function to deflect incident light incident from various directions to within a predetermined emitting-direction range.
The schematic exploded perspective view of a side-light type backlight device including the above-mentioned DIAART™ from MITSUBISHI RAYON Co., LTD is shown in FIG. 48. In this case, a structure is proposed in that on the light incident side, a light deflecting sheet 150 having a triangular-pole grooved plane is arranged to oppose the upper surface 130A, which is a light outgoing plane, of a light guide plate 130, and on the bottom surface 130B opposite to the upper surface 130A of the light guide plate 130, grooves are formed in a direction perpendicular to the extending direction of the triangular-pole prism plane of the light deflecting sheet 150. In such a configuration, light emitted from a light source 121 is reflected by a reflector 135 to enter a side 130s of the light guide plate 130, so that the light outputted therefrom is caused to enter a liquid crystal display panel 136 with desired angular distribution by the grooves formed on the bottom surface 130B of the light guide plate 130; a reflection plate 132 arranged to oppose the bottom surface 130B; and further by the light deflecting sheet 150 (see Patent Document 3, for example).    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 8-136917    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 6-3526    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 11-224516